1. Field of the Invention
The present invention relates to a method and a system for calibrating an input voltage of a voltage controlled oscillator (VCO) and a digital interface used for calibrating an input voltage.
2. Description of the Related Art
Frequency synthesizers are used to generate stable carrier frequency signals in many wireless telephone applications. As a user moves from a cell to a cell, a next cell operates at another frequency and thus requires a highly fast variation in a transmission frequency. Accordingly, the transmission frequency is required to vary. A new transmission frequency may be greatly different, and the transmission frequency must be re-established as a new channel frequency for microseconds so that the user is not affected by the movement from the cell to the cell.
A phase locked loop (PLL) must speedily re-set a phase lock for the new channel frequency to speedily change a frequency. Since a synthesizer does not have a VCO operating throughout a bandwidth of a cellular phone, it is difficult to design the synthesizer. Thus, the VCO must operate in a multiple frequency band or a multiple VCO covering the multiple frequency band must be provided. However, realizing the multiple VCO is costly and difficult.
A wide tuning bandwidth may have a difference between 10% and 15% in a frequency tuning range of the VCO due to a variation in an operation frequency. To solve this problem, a synthesizer using a single VCO operating a multiple band of a concerned spectrum and preventing a variation in a manufacturing process was disclosed in Korean Patent Application No. 2001-0053259. According to the suggested invention, the single VCO of a PLL is digitally controlled so as to tune another range of a frequency.
When an operation frequency of a synthesizer is changed, a reference voltage source is connected to a control input of a VCO to set a frequency of the VCO at an end of a tuning range of the synthesizer. An operation frequency band of the VCO is selectively switched with monitoring the control input of the VCO. When a comparator circuit determines that a tuning voltage of the control input of the VCO is reduced to be less than a reference value, the switching of the operation frequency band of the VCO is controlled, and the VCO obtains a phase locked state in a PLL. According to the prior art, an input power of a VCO is automatically calibrated. However, the input power must be detected from a full band of a frequency whenever an output frequency is changed such that a time delay results.